Prior art processes for the preparation of certain stilbene derivatives, particularly certain 4,4'-bis(substituted-2-benzoxazolyl)stilbenes, can be placed into three types of processes.
The first type of process produces symmetrical compounds which involves the formation of both benzoxazole rings prior to the formation of the stilbene nucleus. Such processes are disclosed in, for example, British Patent 1,072,918; Japanese Patent 66-018,838; and U.S. Pat. Nos. 3,546,217; 3,476,747; and 3,819,615.
The second type of process produces unsymmetrical compounds which involves formation of the stilbene nucleus in between the formation of the two differently substituted benzoxazole rings, or the formation of the stilbene nucleus by the combination of two differently substituted precursors in a selective manner. Such processes are disclosed in, for example, Belgian Patent 659,078; Japanese Patents 70-002,669; 70-002,670; 69-006,978; and U.S. Pat. Nos. 3,586,673; and 3,407,196.
The third type of process produces symmetrical compounds which involves the formation of the stilbene nucleus prior to the formation of both benzoxazole rings. Such processes are disclosed in, for example, Japanese Patents 68-000-811; 66-008,573; 68-028,017; 68-030,193; 70-000,771; South African Patent 68-005,356; French Patent 1,397,727; Belgian Patents 641,426; 651,310; and U.S. Pat. Nos. 3,412,089; 3,575,996; 3,366,575; 3,860,584; 3,585,208.
Dimethyl or diethyl-4,4'-stilbenedicarboxylate has been used as a starting material in the process disclosed in U.S. Pat. No. 3,412,089. This process involves condensing the dialkyl-4,4'-stilbenedicarboxylate with an appropriate o-aminophenol in polyphosphoric acid at 200.degree.-230.degree. C. for 8 hours, cooling to 100.degree. C., pouring into water and filtering. Heating the crude solid in 5% aqueous sodium hydroxide solution, filtering and washing with water gives the final product in reported 95% yield.
It is also known that 2-(p-tolyl)benzoxazole compounds are produced by the direct reaction of methyl-p-toluate with o-aminophenol at an elevated temperature in the presence of a catalytic amount of a tin or titanium compound. However, in the case where dimethyl-4,4'-stilbenedicarboxylate is used instead of methyl-p-toluate in the above direct reaction, we have found that the conversion is low and unsatisfactory for an industrial process. Even when typical industrial solvents are employed that boil below about 200.degree. C. (e.g., benzene, toluene, xylene, chlorobenzene, dimethyl sulfoxide) we have found that conversions remain low.